1. Field of the Invention
The present invention relates to a communication method suitable for application to, for example, a base station and a terminal apparatus in a radio telephone system, and a base station and a terminal apparatus to both of which the communication method is applied.
2. Description of the Related Art
In a mobile communication system such as a radio telephone system or the like, a multiple access is carried out wherein a plurality of mobile stations (subscribers or terminal apparatus) have access to a single base station. In the case of the radio telephone, since a plurality of mobile stations share a single base station, a variety of communication systems have been proposed in order to avoid interference among the respective mobile stations. For example, conventional methods of communication of this type include a frequency division multiple access (FDMA), a time division multiple access (TDMA), a code division multiple access (CDMA) and so on.
According to the TDMA system, each transmission channel allocated to each base station is divided by a predetermined period of time to form a plurality of time slots in one transmission channel, and each of the time slots is allocated to a mobile station to be connected, thereby allowing simultaneous connection to a plurality of mobile stations using one transmission channel.
Further, as another system of communication, the present applicant has proposed a system of communication referred to as a band division multiple access (BDMA) (Japanese unexamined patent publication No. 8-132434 and others). Although the BDMA system will be explained in detail in preferred embodiments of the invention to be described later, briefly, it is a system wherein a plurality of one-transmission bands having a predetermined number of subcarrier signals arranged at predetermined frequency interval are prepared; the signal in each of the transmission bands is segmented at a predetermined period of time to form time slots; and a burst signal is transmitted in the form of a multi-carrier signal which is data distributed to said predetermined number of subcarrier signals intermittently at the cycle of a predetermined number of time slots. The BDMA system exhibits excellent transmission characteristics.
When the TDMA system is applied to a radio telephone system, a terminal apparatus must communicate to a base station in synchronization with a reference timing set by the base station to prevent interference between time slots of each channel. Since the distance between the terminal apparatus and the base station is not constant, even if a signal is transmitted from each terminal apparatus to the base station at the same timing, different propagation delays are caused in the signal transmitted from each terminal apparatus and hence the timing at which the base station can receive these signals varies for each channel.
Thus, there is a need for a process to correct the timing. For example, a process has been performed wherein the base station detects the amount of delay of a signal transmitted from each terminal apparatus from the reference timing and control data is transmitted to the terminal apparatus to shift the transmission timing in accordance with the amount of delay, thereby performing correction such that reception can occur at a constant timing (a time alignment process).
However, such a time alignment process involves bidirectional data transmission and, therefore, it can be carried out only on communication in progress between a base station and a terminal apparatus. For example, it has been impossible to perform a time alignment process on an access request signal transmitted from a terminal apparatus to a base station to request the commencement of communication (access request).
Therefore, in order to prevent transmission of a signal which has not been subjected to a time alignment processing from causing any problem, it can be considered that the number of bits to be transmitted is decreased and a period of a signal burst having the bits allocated on the time axis is set shorter. This has never been applied to a multi-carrier communication to which the present invention is directed. In general, when a method of improving reliability of a signal on a channel for achieving an access right is carried out, only an increase of transmission power per symbol has been attempted. However, this increase of the transmission power per symbol results in interference to other cells in view of the whole cellular system, and hence it is not preferable.